[ruby/optparse] Add rdoc options

Specify the main page and the page directory.  Also in Rakefile,
extract and use the same options from the gemspec file.

https://github.com/ruby/optparse/commit/d182cd60b5

lib/optparse/optparse.gemspec

@@ -23,6 +23,7 @@ Gem::Specification.new do |spec|
   spec.metadata["source_code_uri"] = spec.homepage
 
   spec.files         = Dir["{doc,lib,misc}/**/*"] + %w[README.md ChangeLog COPYING]
+  spec.rdoc_options  = ["--main=README.md", "--op=rdoc", "--page-dir=doc"]
   spec.bindir        = "exe"
   spec.executables   = []
   spec.require_paths = ["lib"]